In recent years, there has been an increasing trend for portable equipment to incorporate a multiaxial physical quantity measuring instrument such as a three-axis acceleration sensor or three-axis magnetic sensor.
FIG. 1 is a block diagram showing a schematic configuration of a three-axis acceleration sensor as a conventional physical quantity measuring instrument, which block diagram corresponds to FIG. 5 shown in Patent Document 1. Three acceleration sensor elements 101, 102 and 103 detect a three-dimensional acceleration vector, and signals Hx, Hy and Hz corresponding to the X-axis component, Y-axis component and Z-axis component of the acceleration are output.
One of the output signals Hx, Hy and Hz is selected by a signal selecting unit 104, and the selected signal is amplified successively by an operational amplifier 105. To the output signal in this case, a noise component N is added. Here, the reference numeral 108 schematically designates the noise component added to the signal.
Thus, the output signal Hx+N, Hy+N or Hz+N is obtained successively. The signal amplified by the operational amplifier 105 is converted to digital data by an A/D converter 106 so that acceleration data corresponding to the X-axis, Y-axis and Z-axis are obtained.
FIG. 2 is a diagram showing the output signal of the A/D converter when the signal selecting unit shown in FIG. 1 makes a selection, and showing the timing of the data acquired when the three-axis acceleration sensor obtains the three-dimensional acceleration data continuously. The upper row shows the signal selected by the signal selecting unit, and the lower row shows the output signal of the A/D converter including the noise component.
The configuration shown in FIG. 1 can provide only a single-axis acceleration data at a time. Accordingly, to acquire the three-dimensional acceleration data continuously, the signal selecting unit must change the signal to be selected with time.
Since the method of successively acquiring the signals as shown in FIG. 2 can use the circuit after the sensor elements such as the operational amplifier 105 and A/D converter 106 in common, it has an advantage of being able to reduce the size of the circuit. Furthermore, if necessary, it can obtain any desired information about acceleration by calculating the signal converted through the A/D converter 106 by the arithmetic processing unit 107.
As a concrete example of the arithmetic processing, there is one that calculates the inclination of the three-axis acceleration sensor with respect to a horizontal plane. Consider the case where the X-axis sensor element 101, Y-axis sensor element 102 and Z-axis sensor element 103 are disposed in the three-axis acceleration sensor as shown in FIG. 3.
The sensor elements 101, 102 and 103 disposed on the individual axes detect gravitational acceleration, and the arithmetic processing unit 107 can obtain the roll angle ψ, the angle which the X-axis forms with horizontal plane, and the pitch angle θ, the angle which the Y-axis forms with the horizontal plane, by the following calculation.
                              ψ          =                                    tan                              -                1                                      ⁢                                          H                X                                            H                Z                                                    ⁢                                  ⁢                  θ          =                                    tan                              -                1                                      ⁢                                          H                Y                                            H                Z                                                                        [                  Expression          ⁢                                          ⁢          1                ]            
As an example of a two-axis or three-axis magnetic sensor, there is one described in Patent Document 2, for example. The Patent Document 2 discloses a three-axis magnetic sensor which places a plurality of Hall elements on edge of a magnetic concentrator, and detects not only the magnetic field in the direction parallel to the magnetic concentrator, but also the magnetic field perpendicular to that direction simultaneously.
FIG. 4 is a block diagram showing a schematic configuration of the magnetic sensor in the Patent Document 2. Assume that the X-axis component of the magnetic field is Hx, the Y-axis component is Hy, and the Z-axis component is Hz. Then the sensor element (A) 201 to sensor element (D) 204 output the following signals, respectively.201: Hx+Hz+n=A 202: −Hx+Hz+n=B 203: Hy+Hz+n=C 204: −Hy+Hz+n=D  [Expression 2]
where n represents a noise component due to the sensor elements.
Since the signals are a mixture of the signal based on the X-axis component and the signal based on the Z-axis component, or a mixture of the signal based on the Y-axis component and the signal based on the Z-axis component, the signal selecting unit 205 selects one of the following combinations so that each component is output separately (the pairs of wires drawn by the same line types in the signal selecting unit 205 shown in FIG. 4 are selected successively).A−BC−DA+B
Although the signals are successively input to the operational amplifier 206 to be amplified, the output signals in this case include an additional noise component N due to the operational amplifier 206. The output of the operational amplifier 206 becomes as follows.(α)=2(Hx+n/√{square root over (2)}+N/2)(β)=2(Hy+n/√{square root over (2)}+N/2)(γ)=2(Hz+n/√{square root over (2)}+N/2)  [Expression 3]
An A/D converter 207 converts the signal output by the operational amplifier 206 to digital data to obtain magnetic field data corresponding to the X-axis, Y-axis and Z-axis.
FIG. 5 is a schematic diagram of the output signal of the A/D converter when the signal selecting unit shown in FIG. 4 makes a selection, and shows timing of the data obtained when the magnetic sensor acquires the three-dimensional magnetic data continuously. The upper row shows the signals selected by the signal selecting unit, and the lower row shows the output signal of the A/D converter into which the noise components are mixed.
If necessary, the arithmetic processing unit 208 can obtain any desired information about the magnetic field by calculating the signals converted by the A/D converter 207.
Thus, as for sensors incorporated into the portable equipment such as the foregoing acceleration sensor and magnetic sensor, there has been an increasing demand for miniaturization as a demand increases for making the portable equipment more multifunctional and smaller. However, as the demand for miniaturization increases on the one hand, the miniaturization of the sensor generally presents a problem of increasing the noise component against the signal on the other. In addition, the portable equipment is susceptible to noise from the other elements because its inside is very highly integrated. These factors together offer an issue of noise reduction of the sensor.
Concerning the noise reduction of the sensor, the Patent Document 2 discloses a method of improving the S/N (signal/noise) ratio using the magnetic concentrator. As for the magnetic field in the direction parallel to the magnetic concentrator, since the magnetic concentrator amplifies its magnetic flux density, the magnetic sensitivity increases, and the S/N ratio improves. In particular, to detect a very week magnetic field such as geomagnetism (30 μT), the improvement of the S/N ratio becomes an important factor. However, when the magnetic concentrator has a much longer length with respect to its height, there is little effect of amplifying the magnetic flux density as to the magnetic field in the perpendicular direction. Accordingly, the magnetic detection sensitivity in the perpendicular direction does not improve, which presents an inevitable problem in that the S/N ratio is worse than that in the direction parallel to the magnetic concentrator. For this reason, a method is used for preventing the imbalance between the S/N ratios in the parallel and perpendicular directions to the magnetic concentrator by suppressing to some extent the improvement in the magnetic sensitivity obtained by using the magnetic concentrator.
In addition, it is not easy to dispose the sensors in an IC in such a manner as to enable detection of the components in the three-axis directions. Generally, when the Hall elements are disposed in the IC, they are placed in parallel with the IC. Since the Hall elements detect the magnetism perpendicular to the magneto-sensitive plane, it is easy for them to detect the magnetism in the perpendicular direction. To detect the magnetism parallel to the IC, however, it is necessary to dispose the elements in the perpendicular direction in the IC, which is not easy. In addition, another problem arises in that the IC becomes higher by the amount of placing the elements perpendicularly to the IC.
For this reason, a method is sometimes used which disposes the sensor elements on the skew, in which case the detection sensitivity is reduced in some axis directions. Accordingly, a method is used which improves the magnetic detection sensitivity in the perpendicular direction by increasing the number of the sensor elements. However, the method presents a new problem of increasing the size of the magnetic sensor.
Patent Document 3 shows a magnetic sensor including first and second magnetic detecting units capable of outputting first and second detection signals with their phases shifted by 90 degrees; a signal processing circuit for digitizing and amplifying the first and second detection signals; and a power supply circuit for intermittently supplying power to the first and second magnetic detecting units and the signal processing circuit. In addition, it includes a noise processing circuit for reducing noise components contained in the first detection signal digitized and output by the signal processing circuit. The Document 3 describes that the noise processing circuit reduces the noise components contained before and after the switching points of the logic levels of the first detection signal in accordance with the logic levels of the digitized first and second detection signals.
Patent Document 4 describes a method of eliminating offsets of Hall elements and their output amplifiers using a chopper. Here in the physical quantity measuring instrument, the offsets, which are caused by the sensor elements or a circuit following the sensor elements, are mentioned as one of the causes of the S/N ratio reduction just as the noise.
Patent Document 5 describes a three-axis magnetic sensor which has a configuration that places a plurality of electrodes on a single Hall element and outputs from the individual electrodes mixtures of the magnetic components in the horizontal direction and perpendicular direction in the same manner as the Patent Document 2; and which detects the magnetic components in the three-dimensional directions simultaneously by adding and subtracting the outputs from the electrodes by operational amplifiers.
However, the foregoing Patent Documents have a problem in that since they detect the physical quantity to be measured for each component individually, the timing of measurement for each component varies from component to component. In addition, the noise reduction method of the foregoing Patent Document 3 is, just like a similar fashion as other noise reduction methods, applied to the physical quantity to be measured or a measuring device dedicatedly. It means, when the physical quantity to be measured or the configuration of a device is changed, the noise reduction method applied to the foregoing Patent Document 3 cannot achieve sufficient effect often.
On the other hand, as for the configuration of the Patent Document 5, since it detects the three-dimensional magnetic components simultaneously, the timing of the measurement is not shifted. However, it has a problem of increasing the circuit scale and current consumption.
In addition, although it has an advantage of general-purpose noise component reduction to use elements or semiconductor components that are resistant to generating noise components, the elements or components are usually expensive and large, thereby making it difficult to reduce the cost and size.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a physical quantity measuring instrument and a signal processing method thereof capable of improving the timing difference in the measurement between the individual components corresponding to the axes. Another object is to provide a physical quantity measuring instrument and a signal processing method thereof capable of reducing the noise components and improving reliability without increasing the circuit scale or cost.
Patent Document 1: Japanese Patent Laid-Open No. 2005-65789.
Patent Document 2: Japanese Patent Laid-Open No. 2002-71381.
Patent Document 3: Japanese Patent Laid-Open No. 2001-4408.
Patent Document 4: Japanese Patent Laid-Open No. 2005-283503.
Patent Document 5: U.S. Pat. No. 6,278,271.